Traveling wave device providing prebunched transverse-wave beam

ABSTRACT

A traveling wave tube having a control grid that backs and fills across the entire cathode emitting surface to form a delay line with the cathode. This delay line causes the electron beam to take on the helical pattern of a transverse wave. The traveling wave tube has a slow wave circuit which interacts with the transverse beam wave. Circuit means is provided for coupling signal energy to both the control grid and to the slow wave interaction circuit in a predetermined amplitude and phase relationship. The control grid is operated with DC cutoff bias. Each peak of the signal propagated by the grid-cathode delay line annuls the cutoff bias locally and forms a moving window, analogous to a moving window of a focal plane shutter, emitting an electron beam only from a local area of the cathode, which local area cyclically traverses the cathode at the signal frequency whereby the emitted beam is properly prebunched in a helical configuration for efficient transverse wave interaction.

United States Patent 11 1 DeSantis et a1.

[ TRAVELING WAVE DEVICE PROVIDING PREBUNCIIED TRANSVERSE-WAVE BEAM [75] Inventors: Charles M. DeSantis, Neptune;

Arthur H. Gottfried, Rumson, both of NJ.

[73] Assignee: The United States of America as represented by the Secretary of the Army, Washington, DC

[22] Filed: Apr. 25, 1972 [21] Appl. No.: 247,456

[52] U.S. Cl SIS/3.5, 315/36, 315/5.29, 315/4 [51] Int. Cl. H0lj 25/34 [58] Field of Search 315/3, 3.5, 3.6, 315/4, 5, 5.27, 5.29

[56] References Cited UNITED STATES PATENTS 3,237,047 2 1966 Webster 315 5.29 3,076,117 H1963 Bo ..315/5.29 X 3,309,557 3/1967 Crowley-Milling 315/4 X 2,698,398 12/1954 Ginzton ...'315/5.27 3,378,718 4/1968 Osepchuk.. 315/3.5 3,614,516 10/1971 Phillips 3l5/3.6 3,346,819 10/1967 Birdsall.. BIS/3.6 3,337,765 8/1967 Chapell.. 3l5/3.6 3,315,118 4/1967 Muller 315/3.6

- LOAD 307 I 1451 Sept. 18, 1973 3,417,280 12/1968 Kantorowiczm. 315 3.6 3,341,733 9/1967 KantOr0WiCz.... 315/3.6 3,316,439 411967 Kluver 315/3.5

Primary Examiner-Eli Lieberman Assistant Examiner-Saxfield Chatmon, Jr. Attorney-Harry M. Saragovitz et a1.

[5 7] ABSTRACT A traveling wave tube having a control grid that backs and fills across the entire cathode emitting surface to form a delay line with the cathode. This delay line causes the electron beam to take on the helical pattern of a transverse wave. The traveling wave tube has a slow wave circuit which interacts with the transverse beam wave. Circuit means is provided for coupling signal energy to both the control grid and to the slow wave interaction circuit in a predetermined amplitude and phase relationship. The control grid is operated with DC cutoff bias. Each peak of the: signal propagated by the grid-cathode delay line annuls the cutoff bias locally and forms a moving window, analogous to a moving window of a focal plane shutter, emitting an electron beam only from a local area of the cathode, which local area cyclically traverses the cathode at the signal frequency whereby the emitted beam is properly prebunched in a helical configuration for efficient transverse wave interaction.

7 Claims, 9 Drawing Figures ADJUSTABLE- AMPLITUDE AND LOAD 42 PHASE CONTROL SIGNAL SOURCE PAIENIED EH 81918 3.760.219

SHEET 1 UF 2 FIG. 1

LOAD 7 1E Mb 8 |0\ (b /I4 1 [I8 [20 l6 IZAV WE V k 241 26 L ADJUSTABLE {.1 AMPLITUDE AND LOAD 42 PHASE CONTROL 54 1; -h 36 L l: 40 f 38 3 SIGNAL SOURCE 1 :4 3 /BEAM i FIG. 3 |2- k FIG. 4

EMISSION Vo h t ELECTRICAL LENGTH OF DELAY LINE TRAVELING WAVE DEVICE PROVIDING PREBUNCHED TRANSVERSE-WAVE BEAM BACKGROUND OF THE INVENTION Longitudinal or space-charge waves and transverse waves can be propagated on an electron beam. Transverse waves include cyclotron waves and synchronous waves. Electron-beam propagated transverse waves have been interacted with circuit waves for amplification at microwave frequencies. In any traveling wave device where interaction of an electron beam and a circuit wave produces either density or spatial variations (called bunching) in the electron beam, proper prebunching of that beam will enhance the DC to RF conversion efficiency. Prebunching obviates the nonamplifying bunching action during the initial portion of beam-wave interaction and as a result, increases the length of interaction region that contributes amplifying action. Better efficiency thus realized, i.e. higher power output for a given input power, permits reduction in size and weight of a radar or communication system.

The ideal condition for synchronous or cyclotron transverse waves is for the longitudinal electron beam to take on a helical configuration, and that the helical configuration transit the interaction region longitudinally, i'n synchronism with the circuit fields.

For the transverse synchronous wave, the beam electrons do not rotate about the tube axis. In order that the beam electrons not rotate duringlongitudinal transit in an axially directed magnetic field, the electrons are injected into the magnetic field, parallel to .the magnetic field. A beam in which the electrons are spatially distributed for synchronous wave interaction might be thought of as a beam obtained from an electron gun positioned off an axis and rotating about the axis while emitting electrons parallel to the axis but with no com- 7 ponent of motion imparted to the electrons normal to the axis.

For the cyclotron wave, the beam electrons rotate about the tube axis at cyclotron frequency during their longitudinal transit under the influence of the magnetic and electric fields. The cyclotron frequency is determined by the longitudinal magnetic field strength. The beam configuration needed for the cyclotron wave might be thought of as a beam from a gun rotating about the axis of the tube and either injecting the electrons obliquely into axially directed magnetic and electric fields or injecting the electrons into a divergent magnetic field that would impart cyclotronmotion to the electrons.

SUMMARY OF THE INVENTION to the grid-cathode delay line in a predetermined phase and amplitude relationship with respect to the signal coupled to the interaction circuit. Signal energy coupled to the grid-cathode delay line is propagated through the delay line. The signal amplitudeon the delay line is selected so that each positive signal peak of the traveling wave on the delay line annuls the DC positive peak. This establishes a moving window for emitting electrons. Theelectron beam emitting window moving along the delay line may be compared to the light transmitting moving window of an activated focal plane shutter. Preferably, the signal propagation time through the delay line is equal to the period of the signal; if not equal, operation would be less than optimal. The grid-cathode delay line either is circularto simulate a rotating gun or is linear and parallel to the tube axis to simulate a gun cyclically traversing a linear path parallel to the .tube axis while emitting electrons oblique to the axis. The novel electron gun and its arrangement inthe traveling wave tube provides a helical electron beam. The electron gun and the electric and magnetic fields are designed so that for both transverse synchronous waves and transverse cyclotron waves, the

of observationperpendicular to the tube axis equals the signal frequency in the circuit for proper interaction with thecircuit wave.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 is a schematic showing of an embodiment of the invention including a traveling wave tube for trans verse waves and associated circuitry;

FIG. 2 is a simplified axial sectional view of an electron gun construction for the traveling wave tube of FIG. 1';

FIG. 3 is a plan view of the grid arrangement seen in direction 3-3 of FIG. 2;

FIGS. 4 and 5 are voltage and current characteristics of the electron gun of FIGS. 2 and 3;

FIG. 6 is an alternative electron gun construction for the traveling wave tube of FIG. 1, shown in perspective and on a larger scale;

FIG. 7 is a schematic showing of a traveling wave tube for transverse cyclotron wave interaction, not including circuitry nor slow wave means shown in FIG. 1;

FIG. 8 is a simplified perspective view of another embodiment for transverse cyclotron wave interaction; and

FIG. 9 illustrates schematically the operation of the embodiment in FIG. 8 looking in direction 9-9 of FIG. 8.

In FIG. 1 there is shown a traveling wave tube 8 having an electron gun 10, for producing a helical electron beam for transverse wave interaction, that includes an emitter cathode 12, a control grid 14 with input and output terminals 14a and 14b and an anode 15, a collector 16, a slow-wave circuit means 18 providing the electron-beam circuit-wave interaction region and an evacuated envelope 20 containing the recited elements. A steady magnetic field II is provided in the tube parallel to the axis of the slow-wave means 18. DC voltage connections between cathode 12 and anode l5, collector l6, and the slow wave means, 18, are omitted.

The electron gun 10 for producing a helical electron beam for transverse wave interaction is shown in FIGS. 2 and 3. Cathode 12 is hollow and cylindrical with an annular emitter surface at one end and is supported coaxially with slow wave means 18. Grid 14 is a single continuous conductor having the configuration of a meander line located in front of and proximate to the annular emitter face, backing and filling across the emitter face around the entire cathode. The ends of the grid are contiguous and provide terminals 140 and 14b. The combination of grid 14 and the cathode 12 are made in accordance with teachings well known in the microwave art to function as a delay line in the frequency band for which the tube is designed. The cathode 12 forms one side of the circularly configured delay line, and the grid 14 forms the other side of the delay line. The delay line is able to serve the dual purpose of propagating a signal wave and blocking electron beam emission when under the influence of negative cutoff DC bias between grid and cathode except in a moving local area where the DC bias is annuled by a positive peak of the propagated signal wave. The delay line has a propagation constantB such that at any instant one full wavelength of a propagated signal wave exists around the circumference of the circular delay line provided the input signal is CW within the frequency band of the delay line. CW as used herein includes pulsed operation. Each positive peak forms a moving window for beam emission, provided the signal amplitude is adequate to annul the cutoff bias.

Grid-cathode cutoff bias potential is established by DC source 24. The DC source 24 is connected in series with an RF isolating resistor 26 between the cathode l2 and the grid terminal 14a. A terminating load 28 for the grid-cathode delay line and a DC blocking capacitor 30 in series, are connected between the terminal 14b of the grid and the cathode. Signal energy input to the grid-cathode delay line and to the slow wave means is supplied by the same signal source 32. The signal source 32 is connected between gridinput terminal 14a and the cathode 12 by DC blocking capacitors 34 and 36. The connection between the cathode and the signal source includes means 38 at reference or ground potential. Isolating resistor 26 essentially blocks signal en- 'ergy through the DC source 24. The described electron gun emits a beam having helical configuration but wherein each electron of the beam traces a rectilinear path. An adjustable amplitude and phase control means 40 couples RF from the signal source to the input end of the slow wave means. The amplified signal is delivered to an RF load 42 connected between the output end of the slow-wave means and reference potential means 38. The signal level from the signal source is adjustable and is used to set the signal amplitude on the grid-cathode delay line. The adjustable amplitude and phase control means is used to adjust the signal energy to the slow wave means relative to the signal in the gridcathode delay line. Conventional traveling-wave-tube electric and magnetic field adjustment means are to be understood. The embodiment described operates in a manner to provide transverse synchronous wave interaction:

FIGS. 4 and 5 show the voltage and current characteristics of the electron gun of FIGS. 2 and 3. In FIG. 4, grid to cathode voltage for one location along the grid-cathode delay line is shown as a function of time. The DC cutoff bias is shown by a broken line. Anywhere along the grid-cathode delay line where the local grid potential exceeds V there is emission. The electrical length of the delay line is such that at any instant, only one wavelength of signal-produced transverse E field exists around the delay line. There is beam emission only at the instantaneous situs of the waveform peak. The waveform shown in FIG. 4 translates cyclically around the delay line at the signal frequency. FIG. 5 shows the current characteristic corresponding to the voltage characteristic of FIG. 4. Both FIGS. 4 and 5 show that only that part of the traveling voltage wave which annuls the cutoff bias voltage and permits a longitudinally directed burst of current to flow from a local area of the cathode annulus and this current burst follows the traveling wave peak around the gridcathode delay line.

FIG. 6 shows an alternative electron gun configuration for the traveling wave tube of FIG. I having concentric cathode 12B, grid 14B, and anode 15B for providing a helical beam similar to that provided by the electron gun of FIGS. 2 and 3. The broken line represents a beam configuration at one instant; it does not represent the path of an emitted electron.

The traveling wavetube 8C shown in FIG. 7 is a modification of the embodiment shown in FIG. 1 and is operable to inject beam electrons obliquely into the axially directed magnetic and electric fields for providing a rotating helical electron beam and accompanying cyclotron wave. The electrons in the beam rotate about the axis. Corresponding elements are identified by ref erence characters corresponding to those in FIG. 1 followed by upper case C. The electron gun 10C includes cathode 12C, control grid 14C and accelerating anode 15C that are essentially the same as the corresponding elements of FIG. 1 and are operable to function in the same manner. Collector 16C, slow-wave circuit means 18C and envelope 20C are the same as in FIG. 1. In this modification however, the electron gun 10C is oriented obliquely to the axis of the slow wave means 18C. The range of injection angle and accelerating electric field parameters are those for directing the emitted electron beam into the interaction region within the slow wave means and also for rotating the beam around the axis of the slow wave means and not around a line parallel to but spaced significantly from the axis of the slow wave means.

In FIGS. 2, 6 and 7, the grid may be an endless conductor and signal energy may be inductively coupled into and out of the grid.

The helical beam produced by the embodiments described is a prebunched beam and is ideally suited to increasing efficiency of transverse-wave interaction operation and it also permits the use of circuits which can further enhance DC to RF conversion efficiency, i.e., coaxial type circuits, where the beam is injected between conductors of a coaxial line. In addition, the described prebunching in transverse wave tubes reduces interaction with unwanted modes.

It is within the scope of this invention for the circular delay line to be constructed for very narrow band. However, since transverse wave interaction can occur over a broad frequency range and since the utility of the equipment is expanded with increased bandwidth, the circular delay line is constructed to have a reasonable bandwidth. A condition for reasonable bandwidth is that the delay line have the same electrical length, over the desired frequency range. If the electrical length of the circular delay line is made one full wavelength, no more than one helical beam is emitted by the cathode at any time and if the signal is CW, a continuous helical beam is emitted for the duration of the CW. If the delay line length is longer than one wavelength, more than one helical beam is emitted. The number of helical beams depends on the number of positive half cylces or peaks around the circular delay line at the same time. If more than one helical beam is emitted at the same time, only one of the helical beams can be in optimum phase with the circuit wave. Adjustment of the electric field voltage(s) optimizes response.

If the electrical length is less than one wavelength, there is a gap in the beam during every period of the signal. While this will introduce harmonics, the effect of the harmonics can be minimized with associated circuitry; the basic operation is the same as where the electrical length is equal to one wavelength.

In the ideal case, the propagation constant, B for the circular delay line is constant and independent of frequency, thus completely satisfying one wavelength electrical length requirement. Information for designing for near constantB is available in the art. Slow wave circuit designs for the control grid such as the meander line shown in the drawings and interdigital line, not shown, approach the ideal case and in addition do not present a significant physical barrier to electron beam propagation from the cathode.

It is also within the scope of this invention for the electron beam emission and delay line means to be constructed in substantially different configurations from those in FIGS. 1, 2, 3, 6 and 7. For example, a linear configuration of this invention is exemplified by the embodiment shown in FIGS. 8 and 9 wherein cathode 12D and control grid 14D are straight or linear counterparts of the circular cathode and control grid of FIG. 1. Their electrical length is one wavelength or less of the signal to be amplified by the tube. Beam accelerating means parallel to and coextensive with the cathode is to be understood and may take the form of a nonintercepting grid construction but is omitted from FIGS. 8 and 9 to simplify the drawing. Tube envelope D is generally cylindrical but has an offset portion near one end that houses the cathode, grid, and beam accelerating means parallel to but offset from the tube axis. The envelope 20D has a gap between its cylindrical portion and its offset portion which is at least coextensive with the cathode for permitting the emitted electron beam to project into the cylindrical portion of the envelope. The axial magnetic field of the tube urges the beam electrons to a circular path around the tube axis; the electron gun distance d and the magnetic field strength are correlated to assure that the electrons assume a circular path around the tube axis. A slow wave circuit means, not shown, is supported in the cylindrical portion of the envelope starting longitudinally beyond the cathode. DC voltage on the slow wave means acts on the beam electrons to change the circular path of the beam electrons to a helical path. Other electric field producing electrodes to accelerate the beam electrons longitudinally are added if required. FIG. 9 shows the path of one emitted electron, looking in the direction of the arrows 9-9 in FIG. 8. The path is representative of that for every electron emitted from any point along the length of the cathode. The signal may be coupled through the grid 14D in either direction. If the signal direction in the grid-cathode delay line is the same as the circuit wave direction there is a longitudinal gap in the electron beam equal to the length of the cathode since beam emission terminates at one end of the oathode and restarts at the other end of the cathode. If the signal direction in the grid-cathode delay line is opposite to the circuit wave direction and longitudinal beam velocity, and the electrical length of the grid-cathode delay line and the longitudinally directed field are properly coordinated the beam can be made substantially continuous.

We wish it to be understood that we do not desire to be limited to the exact details of construction shown and described, for obvious modifications will occur to a person skilled in the art.

What is claimed is:

1. An improved traveling wave device comprising:

longitudinal slow wave means,

means for providing a magnetic field longitudinally of said slow wave means,

a collector adjacent one end of said slow wave means,

an electron gun adjacent the other end of said slow wave means, said electron gun including a cathode having an emitting surface and a grid that together form a delay line, said grid being disposed adjacent the emitting surface and backing and filling across the entire emitting surface and operable in response to a combination of cutoff bias and superposed signal energy propagated by the delay line with peaks for annulling the cutoff bias to emit only from a moving local area that traverses the cathode cyclically, for emitting a prebunched electron beam for transverse wave interactions along the slow wave means, and

means for coupling signal energy from a common source to the electron gun and to the slow wave means.

2. An improved traveling wave device as defined in claim 1 wherein said signal energy coupling means includes phase control means for adjusting the relative phase of signal energy coupled to the electron gun and to the slow wave means.

3. In the improved traveling wave device as defined in claim 1 wherein said slow wave means is a wire conductor in the form of a helix, and the cathode has an 1 annular emitting surface.

surface all around the cathode.

6. An improved traveling-wave device as defined in claim 1 wherein the device has a longitudinal interaction region along said slow wave means and the cathode has a linear emitting surface parallel to and also oblique to the axis whereby electron emission is directed obliquely to the axis, and the grid backs and fills across the entire length of the linear emitting surface.

7. A method of operating a traveling wave tube having slow wave means, electron beam emission means and a beam collector comprising the steps of:

energizing said electron beam emission means while applying cutoff bias thereto:

coupling selected signal energy to be amplified to the slow wave means, wave means to the electron beam emission means for causing the latter to annulling the cutofi bias in a moving local area only, of the electron beam emission means, which local area retraces its path cyclically with the same period as the signal, to emit a prebunchedelectron beam for transverse wave interaction prior to entry of the beam into the interaction region within said slow wave means. 

1. An improved traveling wave device comprising: longitudinal slow wave means, means for providing a magnetic field longitudinally of said slow wave means, a collector adjacent one end of said slow wave means, an electron gun adjacent the other end of said slow wave means, said electron gun including a cathode having an Emitting surface and a grid that together form a delay line, said grid being disposed adjacent the emitting surface and backing and filling across the entire emitting surface and operable in response to a combination of cutoff bias and superposed signal energy propagated by the delay line with peaks for annulling the cutoff bias to emit only from a moving local area that traverses the cathode cyclically, for emitting a prebunched electron beam for transverse wave interactions along the slow wave means, and means for coupling signal energy from a common source to the electron gun and to the slow wave means.
 2. An improved traveling wave device as defined in claim 1 wherein said signal energy coupling means includes phase control means for adjusting the relative phase of signal energy coupled to the electron gun and to the slow wave means.
 3. In the improved traveling wave device as defined in claim 1 wherein said slow wave means is a wire conductor in the form of a helix, and the cathode has an annular emitting surface.
 4. In the improved traveling-wave device defined in claim 1 wherein the control grid is in the form of a meander line.
 5. In the improved traveling wave device defined in claim 1 wherein the electron gun includes said cathode, said grid and an anode that are coaxial cylindrical structures and wherein the cathode has a cylindrical emitting surface directed toward the anode and the grid backs and fills across the cylindrical cathode emitting surface all around the cathode.
 6. An improved traveling-wave device as defined in claim 1 wherein the device has a longitudinal interaction region along said slow wave means and the cathode has a linear emitting surface parallel to and also oblique to the axis whereby electron emission is directed obliquely to the axis, and the grid backs and fills across the entire length of the linear emitting surface.
 7. A method of operating a traveling wave tube having slow wave means, electron beam emission means and a beam collector comprising the steps of: energizing said electron beam emission means while applying cutoff bias thereto: coupling selected signal energy to be amplified to the slow wave means, wave means to the electron beam emission means for causing the latter to annulling the cutoff bias in a moving local area only, of the electron beam emission means, which local area retraces its path cyclically with the same period as the signal, to emit a prebunched electron beam for transverse wave interaction prior to entry of the beam into the interaction region within said slow wave means. 